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    <title>Custom underlay callback</title>
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  <body>
    <p>Should draw a three-colored background, split horizontally at y:2.25, and
    again on the top at x:19Nov:</p>
    <div id="div_g" style="width:600px; height:300px;"></div>

    <div id="status" style="width:100%; height:200px;"></div>

    <script type="text/javascript">
      s = document.getElementById("status");

      g = new Dygraph(
            document.getElementById("div_g"),
            NoisyData, {
              rollPeriod: 7,
              showRoller: true,
              errorBars: true,

              underlayCallback: function(canvas, area, g) {
                // Selecting a date in the middle of the graph. 
                var splitDate = new Date("2006-11-19").getTime();
                var coords = g.toDomCoords(splitDate, 2.25);

                // splitX and splitY are the coordinates on the canvas for (2006-11-19, 2.25).
                var splitX = coords[0];
                var splitY = coords[1];

                // The drawing area doesn't start at (0, 0), it starts at (area.x, area.y).
                // That's why we subtract them from splitX and splitY. This gives us the
                // actual distance from the upper-left hand corder of the graph itself.
                var leftSideWidth = splitX - area.x;
                var rightSideWidth = area.w - leftSideWidth;
                var topHeight = splitY - area.y;
                var bottomHeight = area.h - topHeight;

                // fillRect(x, y, width, height)
                // Top section: y = (2.25, +Infinity)
                // left: (x < 2006-11-19)
                canvas.fillStyle = 'lightblue';
                canvas.fillRect(area.x, area.y, leftSideWidth, topHeight);

                // right: (x > 2006-11-19)
                canvas.fillStyle = 'orange';
                canvas.fillRect(splitX, area.y, rightSideWidth, topHeight);
        
                // Bottom section: y = (-Infinity, 2.25)
                canvas.fillStyle = 'pink';
                canvas.fillRect(area.x, splitY, area.w, bottomHeight);
              }
            }
          );
    </script>
  </body>
</html>
